Knocking noise in rear

If the noise is constant, see if it varies with the speed of the car. That would indicate something in the rear wheels or rear axles. The first thing to check is to make sure all the lug nuts/bolts are tight. At the same time you need to raise the rear wheels off the ground, one at a time if that's what your jack allows, and grab each tire at the top of the tire with one hand, and at the bottom of the tire with the other. And see if there is any play by rocking the tire in and out. There shouldn't be any play. Really loose lug nuts/bolts will give you play, as would bad rear axle bearings. A little play, just noticeable, might be slightly bad rear axle bearings but that usually doesn't produce a noise. Generally they have to be very bad to give a knocking noise.

Another possibility would be a really bad tire, or a bubble in the tire tread. Make sure you don't have a tire tread that is coming apart.

The key is that you said the noise was constant. Bad shocks will give a more random uneven type noise that is related to the car moving/bouncing down the road, and to the variations in the road. You can sometimes duplicate a bad shock noise by bouncing the car up and down pushing on a fender or bumper.

Jc